Corrosion studies of components subjected to nuclear irradiation have indicated that zirconium-based alloys can suffer localized enhanced corrosion in a radiation field if they are positioned adjacent certain stainless steels or platinum, copper, hafnium or yttrium-based alloys. This localized corrosion effect, referred to as "shadow corrosion," occurs when an alloying element or impurity in the adjacent material becomes activated by neutron capture and subsequently emits .beta.-particles. The .beta.-particles impinge the zirconium-based alloy, producing a "shadow" corrosion image of the adjacent body on the alloy, to the extent that the alloys useful life may be limited by both localized general corrosion and an increase in stress due to oxide volume changes.
Within nuclear reactor cores, high strength austenitic stainless steels containing up to six weight percent manganese are used to form the handles of control blades placed between the fuel channels that enclose nuclear fuel bundles. The fuel channels are typically formed from a zirconium-based alloy such as ZIRCALOY-2 or ZIRCALOY-4, and have been found to be prone to shadow corrosion, which appears on the surface of the fuel channels as an image of the stainless steel handle of the adjacent control blade. It has been further determined that neutron activation of the isotope Mn-56 within the austenitic stainless steel handle is a primary source of .beta.-emission via the following nuclear reactions: ##EQU1## The notation "n.sub.th " stands for a thermal neutron, and "n.sub.f " denotes an epi-thermal neutron at the central energy of an absorption resonance. The cross-section for each reaction is about 13-14 barns, and the half-life of Mn-56 is about 2.58 hr.
Manganese is a key austenite stabilizer in stainless steels at a typical level of about two to six weight percent, which is a sufficient quantity of the isotope to produce significant .beta.-flux when such steels are irradiated by neutron flux. For control blade handles formed from high strength austenitic stainless steels such as XM-19, which contains as much as six weight percent manganese, more than about 90% of the emitted .beta.-particles has been determined to originate from the Mn-56 isotope, with maximum energy of 2.85 MeV. This is sufficiently high to ionize the fuel channels near the surface of the handles, creating a mechanism akin to common forms of corrosion, with sufficient .beta.-flux being produced to result in the aforementioned shadow corrosion effect.
One approach to mitigate "shadow" corrosion involves the development of low manganese stainless steels. If manganese is simply removed from certain stainless steels that are used in components subject to a high neutron fluence, the stress corrosion resistance of the components decreases dramatically. The reduction or elimination of manganese would also increase the probability that at least a portion of the stainless steel would undergo the diffusionless martensite transformation to produce martensite, the presence of which is known to reduce the stress corrosion cracking resistance of a stainless steel. Therefore, to compensate for the reduction or loss of manganese, other compensatory austenite-stabilizing alloying elements, such as nickel, carbon and/or nitrogen, are added to the stainless steel.
While lower alloying levels of manganese serves to substantially reduce the shadow corrosion effect, any modification to the composition of a stainless steel for a nuclear reactor core component requires extensive qualification, including complete metallurgical and fabrication evaluation in both laboratory and in-reactor corrosion testing. This is an expensive, time consuming process with an uncertain probability of success. Accordingly, a more efficient approach is desired to reduce shadow corrosion within a nuclear reactor core without requiring any modification of the compositions of its components.